Having been answered completely and thoroughly by Balacertar and Adeptus from a mechanics perspective, I would like to answer your question from a different point of view.
Intent Determines Use
Most magic systems I'm aware of interact with emotions or intentions on some level or another. At a minimum, concentrating to produce the desired result. Given this is effectively positive energy magic, this might easily apply here. If you intend to harm undead, that aggressive emotion and intent shapes the magic differently than the intent to harm allies would, and vice-versa. Because you are assumed to not be able to feel aggressive destruction while you feel supportive healing, you cannot pull off both at once. The effects are both bursts of energy, but of slightly different manifestations
Honestly I house-rule that you can do both at once because I agree, it's two different bursts of the same type of energy, it should be able to do both in one burst, but there is a potential in-world explanation for why it's a choice if you would rather maintain the intended balance of the game.